test_main_invalid_unicode() of test_sys displays stderr on failure

Lib/test/test_sys.py

@@ -510,10 +510,9 @@ class SysModuleTest(unittest.TestCase):
         p = subprocess.Popen([sys.executable, "-c", code], stderr=subprocess.PIPE)
         stdout, stderr = p.communicate()
         self.assertEqual(p.returncode, 1)
-        lines = stderr.splitlines()
-        self.assertGreaterEqual(len(lines), 2)
-        self.assertEqual(b"Unable to decode the command from the command line:", lines[0])
-        self.assertIn(br"UnicodeEncodeError:", lines[1])
+        pattern = b"Unable to decode the command from the command line:"
+        if not stderr.startswith(pattern):
+            raise AssertionError("%a doesn't start with %a" % (stderr, pattern))
 
     def test_sys_flags(self):
         self.assertTrue(sys.flags)